Re: Input on 22" 6.5CM please
you'll lose between 13 and 20 FPS per inch.
Really fast bullets like magnums are the ones that have a big loss in velocity. This is an article I saved.....and you can google "velocity loss per inch of barrel" and find numerous articles and tests people did....all with the same results...going down to a 18 or 20 inch barrel doesn't cost you much in velocity unless you are shooting a really hot round
Read this....interesting shit:
Velocity loss (or gain)
It is worth noting that the velocity figures published in ammunition brochures and reloading manuals are sometimes taken in barrels different in length from those supplied on many rifles. I have seen various estimates of how much velocity is lost (or gained) when a barrel is not the same length as the test barrel in which a cartridge was chronographed. Here are some of them.
The 2001 Edition of the Shooter's Bible states, in the introduction to the Centerfire Rifle Ballistics section, "Barrel length affects velocity, and at various rates depending on the load. As a rule, figure 50 fps per inch of barrel, plus or minus, if your barrel is longer or shorter than 22 inches." However, they do not say what category of load to which this 50 fps average pertains.
Jack O'Connor wrote in The Rifle Book that, "The barrel shorter than standard has a velocity loss which averages about 25 foot-seconds for every inch cut off the barrel. Likewise, there is a velocity gain with a longer barrel." He went on to illustrate this using a .30-06 rifle shooting 180 grain bullets as an example, so his estimate was obviously for rifles in that general performance class.
Other authorities have tried to take into account the different velocity ranges within which modern cartridges operate. The Remington Catalog 2003 includes a "Centerfire Rifle Velocity Vs. Barrel Length" table that shows the following velocity changes for barrels shorter or longer than the test barrel length:
• MV 2000-2500 fps, the approximate change in MV per 1" change in barrel length is 10 fps.
MV 2500-3000 fps, the approximate change in MV per 1" change in barrel length is 20 fps.
MV 3000-3500 fps, the approximate change in MV per 1" change in barrel length is 30 fps.
MV 3500-4000 fps, the approximate change in MV per 1" change in barrel length is 40 fps.

The 45th Edition of the Lyman Reloading Handbook also has a table showing Center Fire Rifle Velocity Vs. Barrel Length. Their figures apply to barrels between 20 and 26 inches in length and agree with the Remington figures. The Lyman table shows the following approximate velocity changes:
• For rifles with muzzle velocities in the 1000-2000 fps range, the change in velocity for each 1" change in barrel length is 5 fps.
For rifles with muzzle velocities in the 2001-2500 fps range, the change in velocity for each 1" change in barrel length is 10 fps.
For rifles with muzzle velocities in the 2501-3000 fps range, the change in velocity for each 1" change in barrel length is 20 fps.
For rifles with muzzle velocities in the 3001-3500 fps range, the change in velocity for each 1" change in barrel length is 30 fps.
For rifles with muzzle velocities in the 3501-4000 fps range, the change in velocity for each 1" change in barrel length is 40 fps.
The 43rd edition of the Lyman reloading Handbook gave some concrete examples of velocity loss for specific calibers and loads. The Lyman technicians chronographed some high velocity cartridges in rifles with barrels ranging in length from 26 inches down to 22 inches with the following results:
• The average loss for the .243 Win./100 grain bullet was 29 fps per inch.
The average loss for the .264 Win. Mag./140 grain bullet was 32 fps per inch.
The average loss for the .300 H&H Mag./220 grain bullet was 25 fps per inch.
For standard high intensity cartridges in the same test, the Lyman technicians chronographed the cartridges in barrel lengths ranging in length from 24 inches down to 20 inches with the following results:
• The average loss for the .270 Win./130 grain bullet was 37 fps per inch.
The average loss for the .270 Win./150 grain bullet was 32 fps per inch.
The average loss for the .300 Sav./180 grain bullet was 17 fps per inch.
The average loss for the .30-06/180 grain bullet was 15 fps per inch.
The average loss for the .35 Rem./200 grain bullet was 11 fps per inch.
After a bunch of disclaimers, the Lyman people concluded, "The rule of thumb is that high speed, high pressure cartridges shed more speed in short barrels than do the low speed, large bore types." It's funny, but that is what I had suspected all along!
